444 research outputs found
Scaling of dynamics with the range of interaction in short-range attractive colloids
We numerically study the dependence of the dynamics on the range of
interaction for the short-range square well potential. We find that,
for small , dynamics scale exactly in the same way as thermodynamics,
both for Newtonian and Brownian microscopic dynamics. For interaction ranges
from a few percent down to the Baxter limit, the relative location of the
attractive glass line and the liquid-gas line does not depend on . This
proves that in this class of potentials, disordered arrested states (gels) can
be generated only as a result of a kinetically arrested phase separation.Comment: 4 pages, 4 figure
The effect of direct interactions on Brownian diffusion
The effect of direct interactions between suspended particles on their diffusion coefficient is investigated starting from the generalized Einstein relation. It is shown that an attractive potential added to the hard core repulsion leads to a decrease of the diffusion coëfficiënt, whereas a repulsive term has the opposite effect. Simple examples of attractive and repulsive potentials are considered in some detail. Using these results the possibility to obtain information on the interaction potential between suspended particles from their diffusion coefficient is discussed
Critical behavior of colloid-polymer mixtures in random porous media
We show that the critical behavior of a colloid-polymer mixture inside a
random porous matrix of quenched hard spheres belongs to the universality class
of the random-field Ising model. We also demonstrate that random-field effects
in colloid-polymer mixtures are surprisingly strong. This makes these systems
attractive candidates to study random-field behavior experimentally.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Polydispersity Effects in Colloid-Polymer Mixtures
We study phase separation and transient gelation in a mixture consisting of
polydisperse colloids and non-adsorbing polymers, where the ratio of the
average size of the polymer to that of the colloid is approximately 0.063.
Unlike what has been reported previously for mixtures with somewhat lower
colloid polydispersity, the addition of polymers does not expand the
fluid-solid coexistence region. Instead, we find a region of fluid-solid
coexistence which has an approximately constant width but an unexpected
re-entrant shape. We detect the presence of a metastable gas-liquid binodal,
which gives rise to two-stepped crystallization kinetics that can be
rationalized as the effect of fractionation. Finally, we find that the
separation into multiple coexisting solid phases at high colloid volume
fractions predicted by equilibrium statistical mechanics is kinetically
suppressed before the system reaches dynamical arrest.Comment: 11 pages, 5 figure
Osmotic pressure induced coupling between cooperativity and stability of a helix-coil transition
Most helix-coil transition theories can be characterized by a set of three
parameters: energetic, describing the (free) energy cost of forming a helical
state in one repeating unit; entropic, accounting for the decrease of entropy
due to the helical state formation; and geometric, indicating how many
repeating units are affected by the formation of one helical state. Depending
on their effect on the helix-coil transition, solvents or co-solutes can be
classified with respect to their action on these parameters. Solvent
interactions that alter the entropic cost of helix formation by their osmotic
action can affect both the stability (transition temperature) and the
cooperativity (transition interval) of the helix-coil transition. A consistent
inclusion of osmotic pressure effects in a description of helix-coil transition
for poly(L-glutamic acid) in solution with polyethylene glycol can offer an
explanation of the experimentally observed linear dependence of transition
temperature on osmotic pressure as well as the concurrent changes in the
cooperativity of the transition.Comment: 5 pages, 3 figures. To be submitted to Phys.Rev.Let
Diffusive Evolution of Stable and Metastable Phases II: Theory of Non-Equilibrium Behaviour in Colloid-Polymer Mixtures
By analytically solving some simple models of phase-ordering kinetics, we
suggest a mechanism for the onset of non-equilibrium behaviour in
colloid-polymer mixtures. These mixtures can function as models of atomic
systems; their physics therefore impinges on many areas of thermodynamics and
phase-ordering. An exact solution is found for the motion of a single, planar
interface separating a growing phase of uniform high density from a
supersaturated low density phase, whose diffusive depletion drives the
interfacial motion. In addition, an approximate solution is found for the
one-dimensional evolution of two interfaces, separated by a slab of a
metastable phase at intermediate density. The theory predicts a critical
supersaturation of the low-density phase, above which the two interfaces become
unbound and the metastable phase grows ad infinitum. The growth of the stable
phase is suppressed in this regime.Comment: 27 pages, Latex, eps
Rayleigh-B\'{e}nard convection in a homeotropically aligned nematic liquid crystal
We report experimental results for convection near onset in a thin layer of a
homeotropically aligned nematic liquid crystal heated from below as a function
of the temperature difference and the applied vertical magnetic
field and compare them with theoretical calculations. The experiments cover
the field range 8 \alt h \equiv H/ H_{F} \alt 80 ( is the
Fr\'eedericksz field). For less than a codimension-two field the bifurcation is subcritical and oscillatory, with travelling- and
standing-wave transients. Beyond the bifurcation is stationary and
subcritical until a tricritical field is reached, beyond which it
is supercritical. The bifurcation sequence as a function of found in the
experiment confirms the qualitative aspects of the theoretical predictions.
However, the value of is about 10% higher than the predicted value and
the results for are systematically below the theory by about 2% at small
and by as much as 7% near . At , is continuous within
the experimental resolution whereas the theory indicates a 7% discontinuity.
The theoretical tricritical field is somewhat below the
experimental one. The fully developed flow above for is
chaotic. For the subcritical stationary bifurcation also
leads to a chaotic state. The chaotic states persist upon reducing the Rayleigh
number below , i.e. the bifurcation is hysteretic. Above the tricritical
field , we find a bifurcation to a time independent pattern which within
our resolution is non-hysteretic.Comment: 15 pages incl. 23 eps figure
Isotropic-nematic phase transition in suspensions of filamentous virus and the neutral polymer Dextran
We present an experimental study of the isotropic-nematic phase transition in
an aqueous mixture of charged semi-flexible rods (fd virus) and neutral polymer
(Dextran). A complete phase diagram is measured as a function of ionic strength
and polymer molecular weight. At high ionic strength we find that adding
polymer widens the isotropic-nematic coexistence region with polymers
preferentially partitioning into the isotropic phase, while at low ionic
strength the added polymer has no effect on the phase transition. The nematic
order parameter is determined from birefringence measurements and is found to
be independent of polymer concentration (or equivalently the strength of
attraction). The experimental results are compared with the existing
theoretical predictions for the isotropic-nematic transition in rods with
attractive interactions.Comment: 8 Figures. To be published in Phys. Rev. E. For more information see
http://www.elsie.brandeis.ed
Minimal Obstructions for Partial Representations of Interval Graphs
Interval graphs are intersection graphs of closed intervals. A generalization
of recognition called partial representation extension was introduced recently.
The input gives an interval graph with a partial representation specifying some
pre-drawn intervals. We ask whether the remaining intervals can be added to
create an extending representation. Two linear-time algorithms are known for
solving this problem.
In this paper, we characterize the minimal obstructions which make partial
representations non-extendible. This generalizes Lekkerkerker and Boland's
characterization of the minimal forbidden induced subgraphs of interval graphs.
Each minimal obstruction consists of a forbidden induced subgraph together with
at most four pre-drawn intervals. A Helly-type result follows: A partial
representation is extendible if and only if every quadruple of pre-drawn
intervals is extendible by itself. Our characterization leads to a linear-time
certifying algorithm for partial representation extension
Gel transitions in colloidal suspensions
The idealized mode coupling theory (MCT) is applied to colloidal systems
interacting via short-range attractive interactions of Yukawa form. At low
temperatures MCT predicts a slowing down of the local dynamics and ergodicity
breaking transitions. The nonergodicity transitions share many features with
the colloidal gel transition, and are proposed to be the source of gelation in
colloidal systems. Previous calculations of the phase diagram are complemented
with additional data for shorter ranges of the attractive interaction, showing
that the path of the nonergodicity transition line is then unimpeded by the
gas-liquid critical curve at low temperatures. Particular attention is given to
the critical nonergodicity parameters, motivated by recent experimental
measurements. An asymptotic model is developed, valid for dilute systems of
spheres interacting via strong short-range attractions, and is shown to capture
all aspects of the low temperature MCT nonergodicity transitions.Comment: 12 pages, LaTeX, 5 eps figures, uses ioplppt.sty, to appear in J.
Phys.: Condens. Matte
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